There's not a server provider on the internet which will care about account numbers. What they usually care about it content, if a site violates their policies, it violates their policies, if it doesn't, it doesn't. You're not going to get a server provider to allow/disallow you based on the number of registered accounts. If they'll care about anything, it'd be the number of active accounts or other such active numbers such as daily/monthly connections/visitors. But I just don't see that being something anyone would care about, you don't pay more/less based on users. I just can't see a situation where a server company would reject a diaper/fetish site which didn't have a lot of users and then go "oh, you have that many registered accounts, well in that case go right ahead and violate our policies!" Unless you have absolutely insane amounts of money to throw at the problem. But that wouldn't be worth it, cause there are so many server providers who have absolutely no problem hosting this type of site.
Dead accounts also would hardly affect a server, the most it'd effect is storage. but CPU and other resources would remain mostly unaffected.